M E 433
Professor John M. Cimbala
Lecture 22
Today, we will:
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Continue definitions and terminology regarding particulate air pollution
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Discuss particle motion – how particles move through the air; equations of motion
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Specifically, discuss buoyancy and weight, drag on particles, and drag coefficient.
Example: Calculation of number concentration from mass concentration
Given: Ben measures the PM2.5 mass concentration of polluted air to be cj = 55 µg/m3. The
average density of the particles is 1250 kg/m3. The mean particle diameter based on mass
is measured to be Dp,am (mass) = 1.8 microns.
To do: Calculate the number concentration of particles, cnumber,j in units of millions of
particles per cubic meter. [Be careful with the units – answer should be between 10 and 50.]
Assume spherical particles.
Solution:
Terminology (from Chapter 3 of Phalen and Phalen, plus other references):
• Aerosol = A suspension of particles in air.
• Hydrosol = A suspension of solid particles in water.
• Cloud = An aerosol volume that behaves as an ensemble.
• Dust = An aerosol of dry solid particles, typically supermicron.
• Fumes = An aerosol of submicron particles, typically small condensed liquid particles from
liquids or molten metals.
• Mist = An aerosol of liquid droplets, typically large (Dp > 20 µm for water).
• Smoke = An aerosol formed by combustion, typically condensed vapors plus solid
(unburned) particles and soot.
• Soot = An aerosol of particles resulting from the incomplete
combustion of hydrocarbons, typically due to lack of sufficient
oxygen (fuel rich combustion), or pyrolisis.
• Smog = An aerosol of “smoke” + “fog”, typically due to combustion.
Smog over Los Angeles, from http://blacklemag.com/technology/science-explains-what-causes-smog/